In recent years, what is called a self-emission display device using self-light emitting elements such as light emitting diodes (LEDs) as display elements has been attracting attention. Among the light emitting elements for using in such a self-emission display device, an organic light emitting diode (OLED) and the like are in the spotlight and they are beginning to be used for the displays of display devices and mobile phones.
Because such light emitting element as OLED emits light by itself, it is advantageous in that the visibility of pixels is higher than liquid crystal displays, a backlight is not needed, and a response rate is fast and the like. Moreover, the luminance of a light emitting element is controlled by a current value flowing through a light emitting element.
Known as driving methods for such a display device using a self-light emitting element are a passive matrix type and an active matrix type. The passive matrix type has a rather simple structure, however, there are such problems as a difficulty in realizing a large and high definition display, therefore developments are more on the active matrix type these days, which controls the current flowing to the light emitting element with a thin film transistor (TFT) provided in a pixel circuit.
In the case of such active matrix display device, there is a problem that a luminance varies because of the change in the current flowing to the light emitting element due to the variation in characteristics of driving TFTs. In the case of the active matrix display device, a pixel circuit 100 employs driving TFTs for driving the current flowing to the light emitting element, however, when the characteristics of these driving TFTs vary, the current flowing to the light emitting element changes, and the luminance varies. In view of such problems, various circuits for suppressing the variation in luminance, in which the current flowing to the light emitting element does not change even when the characteristics of the driving TFTs in the pixel circuit vary, are suggested.    Patent Document 1: Published Japanese Translation of a PCT Application No. 2002-517806    Patent Document 2: International Publication No. WO01-06484    Patent Document 3: Published Japanese Translation of a PCT Application No. 2002-514320    Patent Document 4: International Publication No. WO02-39420
Each of the patent documents 1 to 4 discloses the structure of the active matrix display device, and patent documents 1 to 3 disclose circuit configurations in which a current flowing to the light emitting element does not change due to the variation in characteristics of the driving TFTs disposed in the pixel circuit. Further, the patent document 4 discloses a circuit configuration for suppressing the change of driving current due to the variation of TFTs in a source driver circuit.
FIG. 30 is a circuit diagram showing an example of a conventional active matrix display device disclosed in the patent document 1.
This display device is configured with a plurality of pixel circuits 100 disposed in matrix and a source driver circuit 200 for driving the pixel circuits 100. A signal current having a signal level corresponding to the image data is supplied through a signal line 20 for each pixel, and a driving current which is in proportion with this signal current is supplied to a light emitting element 40 in the pixel circuit 100 from a power supply line 30.
The pixel circuit 100 comprises an OLED 40 which is a current drive light emitting element, a light emitting TFT 52 which switches between ON and OFF corresponding to a control signal of a control line 10c, a select TFT 51 which switches between ON and OFF corresponding to a control voltage of a control line 10b so that a signal current having a current level corresponding to image data supplied to the signal line flows, a driving TFT 50 which supplies driving current from the power supply line 30, a holding capacitor 60 which is connected between the gate and source of the driving TFT 50, a holding TFT 53 which switches between ON and OFF corresponding to a control signal of the control line 10a and selectively connects the gate and drain of the driving TFT 50. Further, a source driver circuit 200 has an image signal input current source 70 which outputs a signal current Ivideo having a signal level corresponding to image data.
The operation of the circuit is explained now.
First, as shown in FIG. 31, the holding TFT 53 and the select TFT 51 are turned ON by the control voltage applied to the control lines 10a and 10b. Then, the signal current Ivideo, which is determined by the image signal input current source 70 flows from the power supply line 30 through the driving TFT 50 and the select TFT 51 as shown by a dotted line in FIG. 31.
At this time, a voltage between the gate and source, Vgs is applied between the gate and source of the driving TFT 50, which is required for the signal current Ivideo to flow. The voltage is stored in the holding capacitor 60, and the current stops flowing to the holding TFT 53 when it reaches the steady state.
Next, as shown in FIG. 32, the holding TFT 53 is turned OFF.
Then the voltage between the gate and source, Vgs is stored in the holding capacitor 60 and this storage voltage Vgs keeps the signal current Ivideo flowing to the driving TFT 50. After that, the select TFT 51 is turned OFF and a light emitting TFT 52 is turned ON as shown in FIG. 33. Thus, the signal current Ivideo starts flowing to the OLED 40.
Here, a voltage between the drain and source of the driving TFT 50, Vds has a different value between the cases of FIG. 32 and FIG. 33. In the case where the driving TFT 50 operates in a saturation region, however, the same current Ivideo flows as long as a voltage between the gate and source, Vgs is the same even when a voltage between the source and drain, Vds changes. Therefore, it is advantageous that the current flowing to the OLED is constant even when the current voltage characteristic is changed due to the deterioration of the characteristics of OLED, thus the luminance is not easily deteriorated.
Also, constant current flows as long as the voltage stored in the holding capacitor 60 is constant, even when a voltage between the drain and source of the driving TFT changes. Therefore, a problem that the signal current is changed due to manufacturing variations of the driving TFT 50 can be avoided.
Abovementioned examples relates to the technology for correcting the change in signal current due to the variation of the OLED 40 and the driving TFT in the pixel circuit, however, the same problem occurs in the source driver circuit.
Patent document 4 discloses a circuit configuration for preventing the change in signal current due to the manufacturing variations of TFTs in a source driver circuit.